This invention relates to a circuit arrangement suitable for operating a low-pressure mercury discharge lamp by means of a high-frequency current, comprising
circuitry for generating the high-frequency current from a supply voltage, and
a modulator for the substantially square-wave modulation of the amplitude of the high-frequency current with a modulation frequency f.
Such a circuit arrangement is known from U.S. Pat. No. 4,219,760.
During lamp operation, a substantially square-wave modulated high-frequency voltage is present across the low-pressure mercury discharge lamp, to be referred to hereinafter as the lamp. The frequency and phase of the substantially square-wave modulation of the high-frequency voltage across the lamp are equal to the frequency and phase of the substantially square-wave modulation of the high-frequency current. The lamp is re-ignited by means of the high-frequency voltage, which then acts as the reignition voltage, at the start of every square wave of the substantially square-wave modulated high-frequency voltage. After this, the amplitude of the high-frequency voltage decreases to a substantially constant lamp-dependent value owing to the fact that the lamp becomes conductive, and the high-frequency voltage causes the high-frequency current to flow through the lamp during the square wave. In the remaining portion of each cycle of the substantially square-wave modulation of the high-frequency voltage there is substantially no voltage across the lamp and substantially no current flows through the lamp. The lamp is dimmed in that a duty cycle .delta. of the substantially square-wave modulation of the high-frequency voltage is adjusted. It was found that the luminous efficacy is comparatively high and substantially independent of the luminous flux with this dimming method.
When a circuit arrangement according to the prior art is used, the amplitude of the re-ignition voltage assumes a comparatively high value at the beginning of every square wave, so that the lamp is quickly re-ignited. This comparatively high amplitude of the re-ignition voltage, however, adversely affects the life of the circuit arrangement and also causes a momentary light pulse having a high amplitude. Since these light pulses also contain infrared light, they constitute an interference signal for, for example, infrared systems such as infrared remote control systems or audio transfer systems.